JP2014115084A - Gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas sensor which can further increase sensitivity of the sensor by removing attenuation of a surface acoustic wave by a sensitive film in a SAW gas sensor.SOLUTION: Ion bonding property of a piezoelectric crystal substrate is evaluated, and by selecting a crystal whose ion bonding property is equal to or larger than a certain index (preferably 50% or higher), polarity gas is adsorbed without using a sensitive film causing attenuation of a surface acoustic wave to increase an interactive distance between the surface acoustic wave and the gas. That is, a piezoelectric crystal substrate having structural elements that have ion bonding property of 50% or higher is allowed to propagate a surface acoustic wave, and a change of sound speed or attenuation of the surface acoustic wave which is caused by adsorbing polarity gas molecules on the surface of the piezoelectric crystal substrate is used.

Description

  In order to increase the sensitivity of a gas sensor for polar gas, the present invention evaluates the ion binding property of a piezoelectric crystal substrate, and selects a crystal having a certain index or higher (preferably 50% or higher) to thereby generate a surface acoustic wave. The present invention relates to a novel gas sensor construction method characterized in that polar gas is adsorbed without using a sensitive film that causes attenuation, and the interaction distance between the surface acoustic wave and the gas is increased.

  In order to measure volatile organic compounds (VOC) in the environment, it is necessary to reduce power consumption and weight, and surface acoustic waves (SAW) that can measure gas at room temperature; ) A sensor is useful (see, for example, Non-Patent Document 1).

  The SAW sensor is manufactured by forming a sensitive film on the SAW propagation path in the SAW element. Here, the sensitive film changes the sound speed and attenuation of the SAW by absorbing or adsorbing gas and changing its density, elastic modulus, viscous attenuation, conductivity, and the like. The sensitivity of the SAW sensor can be increased by increasing the interaction distance between the SAW and the gas and improving the gas detection efficiency in the sensitive film.

  Regarding the enhancement of sensitivity based on the interaction distance between the SAW and the gas, the planar SAW sensor has a limit in increasing the interaction distance due to diffraction. On the other hand, the spherical SAW sensor (ball SAW sensor) developed by the group to which the inventors belong belongs to the phenomenon that SAW's natural collimated beam circulates multiple times, and the interaction distance is significantly increased compared to the planar sensor. Therefore, it is useful for increasing the sensitivity (see, for example, Patent Documents 1, 2, 3, and Non-Patent Document 2).

  Regarding the improvement of gas detection efficiency by the sensitive film, in order to achieve high sensitivity to polar VOCs such as alcohol at room temperature, it is necessary to provide a sensitive film and absorb or adsorb the gas on the sensitive film. A stationary phase substance used in a gas separation column of a gas chromatograph is useful as a sensitive membrane material (see, for example, Non-Patent Document 1), and it is generally known that a polar gas is easily retained by a polar stationary phase substance ( For example, refer nonpatent literature 3). For example, in the measurement of n-butanol, a sensitive film prepared so that the hydrophilic group in the amphiphile (surfactant), which is a stationary phase for polar gas analysis, is oriented on the outermost surface, It was useful for high sensitivity (for example, refer nonpatent literature 4). However, the provision of the sensitive film increases the attenuation of the SAW, so that the propagation distance cannot be increased, and there is a problem that higher sensitivity is hindered.

Here, as the SAW element, quartz or Langasite (La ₃ Ga ₅ SiO ₁₄ ; LGS) whose electromechanical coupling coefficient is about three times larger than that of quartz is used (for example, see Non-Patent Document 5). Both of these crystals have an oxygen tetrahedron of Si, but LGS also has an oxygen polyhedron of La and Ga (see, for example, Non-Patent Document 6).

  Both quartz and LGS can be hydrophilized by ultraviolet irradiation (see, for example, Non-Patent Document 7). The hydrophilized surface is useful for the adsorption of polar VOCs, but no SAW sensor using this instead of a sensitive membrane has been reported. Moreover, the difference in the adsorption characteristics of polar VOC between quartz and LGS is still unclear.

The polarity of the crystal surface depends on the nature of the interatomic bonds constituting the crystal. The polarity of the bond can be evaluated by ionic bondability, and the ionic bondability of the interatomic bond between atom A and atom B is defined by formula (1) (for example, see Non-Patent Document 8).
Here, χ _A and χ _B are the electronegativity of atom A and atom B, respectively.

Japanese Patent No. 3974765 Japanese Patent No. 3974766 Japanese Patent No. 4143296

D.S. Ballantine, R. M. White, S. J. Martin, A. J. Ricco, E. T. Zellers, G. C. Frye, and H. Wohltjen, “Acoustic Waves Sensors Theory, Design, and Physico-Chemical Applications”, Academic Press, 1996 K. Yamanaka, S. Ishikawa, N. Nakaso, N. Takeda, DY. Sim, T. Mihara, A. Mizukami, I. Satoh, S. Akao, and Y. Tsukahara, ”Ultramultiple Roundtrips of Surface Acoustic Wave onSphere Realizing Innovation of Gas Sensors ”, IEEE Trans. UFFC., 2006, 53, p.793-801 Edited by Japan Society for Analytical Chemistry Gas Chromatography, “Capillary Gas Chromatography”, Asakura Shoten, 1997 B. Wyszynski, P. Somboon, and T. Nakamoto, “Highly sensitive QCModor-sensors functionalized with self-assembled lipid-derivatives and GCmaterials”, Proc. IEEE Sensors 2008 Conference, 2008, p.1552-1555 A. A. Kaminski, I. M. Silvestrova, S. E. Srkisov and G. A. Denisenko, “Investigation of Trigonal (La1-xNdx) Ga5SiO14 Crystals”, Phys. Stat. Sol., 1983, A80, p.67 J. Bohm, RBHeimann, M. Hengst, R. Roewer, J. Schindler, “Czochralski growth and characterization of piezoelectric single crystals with langasite structure: La3Ga5SiO14 (LGS), La3Ga5.5Nb0.5O14 (LGN), and La3Ga5.5Ta0. 5O14 (LGT) Part I ”, J. Crystal Growth, 1999, 204, p.128-136 R. John and W. John, “UV / Ozone Cleaning of Surfaces”, IEEE Trans. Parts, Hybrids, and PackagingPHP-12 (4), 1976 B. S. Mitchell, “An Introduction to Materials Engineering and Science for Chemical and Materials Engineers”, Willey-Intercience, 2003, 11

  An object of the present invention is to provide a gas sensor capable of removing the SAW attenuation due to the sensitive film in the surface acoustic wave gas sensor and further increasing the sensitivity of the sensor.

  The present invention evaluates the ionic bonding properties of a piezoelectric crystal substrate, and selects a crystal that has a certain index or higher (preferably 50% or higher), so that the polarity can be obtained without using a sensitive film that causes surface acoustic wave attenuation. Provided is a novel gas sensor construction method characterized in that gas is adsorbed and the interaction distance between the surface acoustic wave and the gas is increased.

  That is, the gas sensor according to the present invention is produced by propagating a surface acoustic wave to a piezoelectric crystal substrate having a component having an ion binding property of 50% or more and adsorbing polar gas molecules on the surface of the piezoelectric crystal substrate. A change in sound velocity or attenuation of the surface acoustic wave is used.

  The detailed description of the present invention will be given by taking quartz and LGS as examples. In formula (1), Si− calculated by substituting the electronegativity O: 3.44, Si: 1.90, Ga: 1.81, and La: 1.10 of each element constituting quartz and LGS. Table 1 shows ionic bonds in O, Ga-O, and La-O bonds (bonding). The ionic bondability of La—O and Ga—O bonds contained in LGS is higher than that of Si—O bonds, and in particular, La—O bonds are significantly higher. As described above, LGS has a high surface ion polarity due to its high ionic bondability, and is strongly advantageous as a sensor by strongly adsorbing polar VOC such as butanol.

  In addition, once polar gas is adsorbed on such a surface, the surface with high sensitivity is covered with this adsorption layer, and the sensitivity is lowered. In the present invention, a method for improving the sensitivity by removing the adsorption layer by ultraviolet irradiation in an oxygen atmosphere is also presented. That is, the gas sensor according to the present invention is preferably irradiated with ultraviolet light by exposing the surface of the piezoelectric crystal substrate to an oxygen atmosphere before use. Moreover, it is preferable to expose the surface of the used piezoelectric crystal substrate to an oxygen atmosphere and irradiate ultraviolet light to expose the surface having high ion binding property and regenerate sensitivity.

  As a method for increasing the interaction distance between the SAW and the gas, a ball SAW element may be used. That is, in the gas sensor according to the present invention, the piezoelectric crystal substrate may be spherical. However, it goes without saying that the present invention is not limited to using an LGS crystal, does not necessarily require ultraviolet irradiation, and is not limited to using a ball SAW sensor.

  ADVANTAGE OF THE INVENTION According to this invention, the highly sensitive gas sensor with respect to polar gas can be provided.

It is a perspective view which shows the crystal structure of (a) quartz and (b) Langasite (LGS). It is a side view which shows the typical effect of the ultraviolet-ray (UV) irradiation with respect to the surface of a crystal | crystallization or LGS. (A) Crystal after organic solvent cleaning, (b) Crystal after acid cleaning, (c) Crystal after UV irradiation, (d) LGS after organic solvent cleaning, (e) LGS after acid cleaning, (f) It is a side view which shows the measurement result of the contact angle of deionized (DI) water in LGS after UV irradiation. (A) Quartz after UV irradiation, (b) Crystal after butanol immersion, (c) LGS after UV irradiation, (d) Side view showing measurement results of contact angle of DI water in LGS after further butanol immersion It is. It is a perspective view which shows the principle of a ball | bowl SAW sensor. It is a schematic diagram which shows the experimental apparatus for the low concentration gas measurement of the gas sensor of embodiment of this invention. 6 shows (a) change in delay time of crystal ball SAW sensor when measuring butanol diluted with methanol, and (b) thermal conductivity detector (TCD) of desktop gas chromatograph (GC). ). FIG. 7 is a graph showing (a) a change in delay time of a ball SAW sensor made of LGS and (b) a TCD signal of a desktop GC when measuring butanol diluted with methanol by the experimental apparatus shown in FIG. 6. A ball made of LGS when the butanol having a concentration of (a) 4500 ppb, (b) 1800 ppb, (c) 900 ppb, (d) 450 ppb, (e) 180 ppb, (f) 90 pp was measured by the experimental apparatus shown in FIG. It is a graph which shows the delay time change of a SAW sensor. It is a graph which shows the relationship between the delay time change measured with the crystal | crystallization and LGS ball | bowl SAW sensor of UV irradiation of the gas sensor of embodiment of this invention, and butanol gas concentration.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
As an example, paying attention to the conventionally used quartz and the recently used LGS, the usefulness of the sensor construction method using the difference in these ion binding properties will be shown. Note that a resonator using a reflector is known as a normal surface acoustic wave sensor that increases the interaction length. In this embodiment, the response to a low-concentration gas is obtained by using the SAW multi-turn phenomenon. By using a ball SAW sensor that amplifies in this way, it is shown that the highest sensitivity in the SAW sensor, a detection limit of 5.8 ppb, can be achieved.

The water contact angle measurement is shown as a method for verifying the difference in adsorption between quartz and LGS. As the substrate, mirror-polished ST-cut quartz and 48.5 ^o Y-26 ^o X LGS were used. The crystal structure of quartz and langasite is shown in FIG. These crystals contain a common tetrahedral of SiO ₄ , but LGS further includes La and Ga oxygen polyhedra.

First, as an organic cleaning for the purpose of removing organic contaminants attached to the surface of crystal or LGS, ultrasonic cleaning is performed twice for 5 minutes each in the order of acetone, isopropyl alcohol, and deionized (DI) water. I went one by one. Also, as acid cleaning, after immersion for 20 minutes (piranha etching) in a mixed solution of concentrated sulfuric acid and hydrogen peroxide solution in a volume ratio of 1: 2, the inside of the beaker is replaced with DI water 6 times and rinsed, The ultrasonic cleaning for 5 minutes was performed twice. Next, UV irradiation was performed to make the surface hydrophilic. The general effect of UV irradiation is shown in FIG. The low pressure mercury lamp includes UV with wavelengths of 254 nm and 185 nm. When these UVs are generated in an oxygen atmosphere, UV of 185 nm generates ozone from oxygen molecules, and UV of 254 nm decomposes ozone to generate active oxygen atoms, and many organic substances that constitute substrate contaminants. (Organic contamination). Accordingly, the chemical reaction between the active oxygen and the organic substance proceeds, and the organic substance is volatilized and removed as CO ₂ or H ₂ O. In order to verify the effect of UV irradiation, the substrate that had been subjected to organic cleaning was irradiated with UV for 30 minutes by a low-pressure mercury lamp (28 mW / cm ² ).

  FIG. 3 shows the result of measuring the contact angle by dropping 1 μl of deionized water on the substrate treated by each method. 3A to 3C show the case of quartz, and FIGS. 3D to 3E show the case of LGS. In the organically cleaned sample, the crystal contact angle was θ = 12 ° (FIG. 3A), and LGS was θ = 17 ° (FIG. 3D). On the other hand, in the acid-washed sample, quartz was θ = 0 ° (FIG. 3B), and LGS was θ = 25 ° (FIG. 3E). Next, in the sample irradiated with UV after organic cleaning, both quartz and LGS were θ = 0 ° (FIGS. 3C and 3F). Thus, it has been found that it is not easy to set the contact angle of LGS to θ = 0 °.

  Next, the UV-irradiated crystal and the LGS substrate are dipped in n-butanol (butanol), which is frequently used as a polar VOC indicator substance, and then dried by a spin dryer. From the change in contact angle as shown in FIG. Adsorbability was evaluated. 4A and 4B show the case of quartz, and FIGS. 4C and 4D show the case of LGS. In the case of quartz, the contact angle did not change at θ = 0 ° (FIG. 4B) even after butanol immersion and drying, but in LGS, the contact angle increased to θ = 19 ° (FIG. 4D). Therefore, it was found that butanol did not adsorb on the quartz surface but was removed by centrifugal force, but strongly adsorbed on the LGS surface and not removed by centrifugal force, increasing the contact angle.

  Next, an LGS ball SAW sensor was made hydrophilic by UV irradiation for 30 minutes after organic cleaning. As a comparison object, a crystal ball SAW sensor was constructed by the same processing.

The principle of the ball SAW sensor is shown in FIG. The great circle perpendicular to the Z-axis of the piezoelectric crystal sphere is defined as the equator. When an interdigital transducer (IDT) whose opening length is expressed by the formula (2) is produced at the equator, the SAW diffraction and the focusing by the spherical surface are balanced and the naturally collimated SAW travels on the equator. Propagate.
Here, λ represents the wavelength of SAW, and D represents the diameter of the sphere.

  This SAW propagates for a long distance because it is not scattered by the support portion of the sphere. When the sensitive film in the propagation path reacts with gas molecules, the sound speed and attenuation of the sensitive film change due to changes in mass load and viscoelasticity, so that the delay time and amplitude of the SAW excited by the pulse signal or burst signal change. Here, a crystal surface hydrophilized by UV irradiation is used. Finally, since the delay time and the change in amplitude increase in proportion to the number of turns, the sound speed and attenuation change of the sensitive film can be measured with high sensitivity by measuring the SAW after multiple turns.

  In order to produce a low-concentration butanol gas with good reproducibility, butanol diluted with methanol was measured by separation using a bench-top gas chromatograph (GC). A schematic diagram of the experimental apparatus is shown in FIG.

The mixed sample gas injected into the GC gas line is transported to a 100% polydimethylsiloxane stationary phase open tube column (inner diameter 0.32 mm, length 5 m) by He carrier gas, and each component has solubility in the stationary phase. Are separated in terms of time. These are detected by a ball SAW sensor and a thermal conductivity detector (TCD), and a chromatogram is obtained by plotting the response of each detector on the vertical axis with time on the horizontal axis. Here, the carrier flow rate in the column was 6 ml / min, and the column and the ball SAW sensor were kept at 35 ^° C. by a GC oven. TCD was used at 200 ^° C. The ball SAW sensor has a diameter of 3.3 mm, and has an opening of 230 μm, a line and space of 4 μm, and 10 pairs of IDTs in a z-axis cylinder. The SAW excitation and the measurement of the change in the delay time of the circulating wave were performed with a digital quadrature detector. The number of measurement laps was determined so that the response S / N was maximized.

  The result of the quartz ball SAW sensor irradiated with UV for 30 minutes is shown in FIG. 0.1 μl of butanol diluted to 1/100 was injected and measured at a split ratio of 80. The butanol gas concentration was 9 ppm at the peak. Here, since it is considered that the pressure loss of the carrier gas mainly occurs in the column in the GC, the concentration was evaluated using the gas equation of state on the assumption that the sensor is at atmospheric pressure.

  FIG. 7A shows a change in delay time measured using the 15th round wave of the ball SAW sensor, and a butanol peak was measured following methanol as a solvent. The positive delay time change due to the gas is due to the mass load due to adsorption. The inserted graph is an enlarged view of a portion surrounded by a broken line. Noise was measured by applying an FFT filtering in a frequency range (0.17 Hz to 1.7 Hz) having a large influence on the peak measurement with respect to the baseline chromatogram to obtain an RMS amplitude. As a result, 9.0 ppm of butanol was measured at S / N = 170. On the other hand, a clear peak was not obtained in the TCD signal shown in FIG. Therefore, it was confirmed that the hydrophilic surface can detect butanol.

  FIG. 8 shows the result of a similar experiment performed on the LGS ball SAW sensor. Since 0.5 μl of butanol diluted 1/1000 was injected and measured at a split ratio of 80, the concentration at the peak was 4.5 ppm. FIG. 8A shows a change in delay time measured using the 40th round wave of the ball SAW sensor. In the LGS sensor, a larger adsorption response was observed than in the case of quartz. Since this response is S / N = 530, the LGS sensor was able to detect 4.5 ppm butanol at a higher S / N than the quartz sensor. The tailing of the desorption response is considered to be caused by the strong butanol adsorption phenomenon on the hydrophilic LGS surface shown by the contact angle measurement.

  Next, the result of measuring the adsorption response using a lower concentration of butanol is shown in FIG. Butanol with a peak concentration of 180 ppb was clearly detectable. Furthermore, 90 ppb butanol could also be detected. Thus, the very sensitive adsorption response in the LGS sensor is considered useful for the initial detection of dangerous polar gases.

  In another UV irradiation LGS sensor, when 1.8 ppm butanol was continuously measured 20 times, the response was halved. In addition, if the GC carrier gas was stopped after measuring 50 ppm of butanol for 1 day, no response to the same concentration of butanol was observed. This is considered to be because butanol adsorbed on the GC pipe, column, sensor cell, etc. was gradually released and continued to be adsorbed on the LGS sensor. However, when UV irradiation was performed on a sensor in which no response was observed, a response similar to that before deterioration could be reproduced. Therefore, it is considered that the LGS sensor irradiated with UV can regenerate the sensitivity by UV irradiation in an oxygen atmosphere even if the sensitivity decreases due to excessive adsorption of polar VOC.

  FIG. 10 shows the butanol gas concentration dependence of the delay time response measured with a ball SAW sensor of UV-irradiated quartz (UV-irradiated quartz) and LGS (UV-irradiated LGS). Here, the result of an LGS sensor (Siponate LGS) in which a surfactant is formed by an off-axis spin coating method capable of producing a low-damping organic sensitive film on the ball SAW element is also described as a comparison with the prior art. This surfactant is sodium dodecylbenzenesulfonate and is a stationary phase material used for separation of polar gases in GC. For the purpose of attaching the hydrophobic group of the surfactant to the sensor surface and exposing the hydrophilic group, a film was formed on an LGS sensor treated with hexamethyldisilazane using a 3 wt% toluene solution under conditions of 3000 prm and 20 s.

  The solid line in FIG. 10 is a proportional relationship obtained from the response giving the highest sensitivity in each sensor, and the response amount at a concentration of 1 ppm represents the sensitivity. The dashed line is twice the value of RMS noise. The detection limit is represented by the concentration at the intersection of the solid line and the dotted line. The hydrophilized LGS sensor had higher sensitivity and lower noise than the quartz sensor. Note that both the LGS sensor and the quartz sensor showed a behavior in which the response was saturated with a high concentration of butanol. On the other hand, in the sensor using the sensitive film, the response was not saturated, but the sensitivity was lower by one digit or more than that of the LGS sensor. Table 2 shows the detection limits achieved with each sensor including UV unirradiated quartz and LGS sensors. The lowest detection limit of 5.8 ppb was achieved with a LGS sensor hydrophilized by UV irradiation. Therefore, it was shown that this sensor can detect butanol of ppb order.

Claims (4)

  Propagation of a surface acoustic wave to a piezoelectric crystal substrate having a component having an ion binding property of 50% or more, and adsorbing polar gas molecules on the surface of the piezoelectric crystal substrate results in the sound velocity or attenuation of the surface acoustic wave. A gas sensor using change.   2. The gas sensor according to claim 1, wherein the surface of the piezoelectric crystal substrate is exposed to an oxygen atmosphere and irradiated with ultraviolet light before use.   The gas sensor according to claim 1 or 2, wherein the surface of the used piezoelectric crystal substrate is exposed to an oxygen atmosphere and irradiated with ultraviolet light to expose the surface having high ion binding property and to regenerate sensitivity. 4. The gas sensor according to claim 1, wherein the piezoelectric crystal substrate has a spherical shape.